1. Technical Field of the Invention
The present invention relates generally to a multi-color printer, and more particularly concerns an improved structure of a multi-color electrophotographic printer which is designed to be compact and to provide for easy maintenance while assuring machine reliability and performance as well as copy quality.
2. Background Art
In conventional color electrophotography, a multi-colored image may be formed by superimposing yellow, magenta, cyan, and black toner images in registration to a transfer member in two different ways. One is referred to as a transfer drum system wherein different color toner images are sequentially formed on a single photoconductor and then transferred in registration to a transfer member wrapped about a transfer drum. The second is referred to as a successive transfer system wherein different color toner images formed on respective photoconductors are transferred in sequence to a transfer member carried by a belt.
Japanese Patent First Publication No. 1-252982 teaches a conventional transfer drum system which, as shown in FIG. 1, includes a photoconductor 1, a charging unit 2, a developing station 3, a transfer drum 4, and a photoconductor cleaner 5. The developing station 3 includes a Y-developer 6 for forming a yellow toner image, an M-developer 7 for forming a magenta toner image, a C-developer 8 for forming a cyan toner image, and a Bk-developer 9 for forming a black toner image. The developer station 3 rotates to move each developer in sequence into engagement with the photoconductor 1.
In operation, the photoconductor 1 is rotated counterclockwise to have its surface charged uniformly by the charging unit 2. Subsequently, in response to a yellow image-forming signal, a laser beam 10 is radiated to the photoconductor 1 to form an electrostatic latent image which is, in turn, developed by the Y-developer 6 to form a yellow toner image. One copy sheet is fed from a sheet supply station 11 and then wrapped about the periphery of the transfer drum 4 with its leading edge being held by a claw 12 until the yellow toner image formed on the photoconductor 1 is advanced into engagement with the transfer drum. The rotation of the transfer drum 4 is so timed that a specified portion of the copy sheet wrapped about the transfer drum may coincide with the yellow toner image.
The yellow toner image on the photoconductor 1 is transferred to the copy sheet under activities of a transfer charging unit 13. After the image transfer,. the surface of the photoconductor 1 is cleaned by the cleaner 5 for subsequent image transfer operation. Thereafter, magenta, cyan, and black toner images are sequentially formed in the same manner.
The radiation of the laser beam 10 to the photoconductor 1 is so timed that a toner image on the photoconductor may be in registration with a toner image previously transferred to the copy sheet. In this manner, a multi-colored image is printed on the copy sheet wrapped about the transfer drum 4. The image-printed copy sheet is stripped off the transfer drum 4 by a stripper 14 which, in turn, is advanced to a fixing station 16 through a transport path 15.
The above prior art system, however, has suffered from the following drawbacks. The transfer drum 4 needs to have a large enough diameter to wrap the copy sheet therearound and its structure is complex, resulting in a bulky apparatus. In addition, stiff paper such as post cards or thick copy sheets cannot be used because they are difficult to wrap about the transfer drum. Since the transfer drum 4 is separate from the cleaner 35, it is necessary to maintain them individually. Further, the transfer drum has a limited lifetime, and it is difficult to know automatically as to whether the transfer drum has exceeded its lifetime or not. Moreover, the transfer drum 4 constantly contacts the photoconductor, which will cause the photoconductor 1 to be damaged upon replacement of the transfer drum 4 or to be degraded prematurely on exposure to intense light entering from the outside during the replacement.
Japanese Patent First Publication No. 1-250970 discloses a color image forming apparatus using the successive transfer system, as described above, in which four image-forming stations each including a photoconductor, a developer, and a scanner are arranged, and a copy sheet carried on a belt passes through a transfer station provided beneath each photoconductor to form a colored toner image. This prior art apparatus eliminates the need for a transfer drum, however, it requires developers, such as a laser unit, of a number corresponding to the number of colors used, resulting in a complex and expensive arrangement. In addition, the transfer stations are arranged away from one another, yielding positional or angular misalignment thereof. This will cause colors to be shifted, adversely affecting copy quality.
Japanese Patent First Publication No. 2-212867 exemplifies a color printer wherein different color toner images formed in sequence on a photoconductor are temporarily placed in registration on an intermediate transfer member and then transferred together to a copy sheet. In this printer, a plurality of developers need to be arranged around the photoconductor for forming all the different color toner images on the same photoconductor. Therefore, a large-sized photoconductor or a belt-like photoconductor is required which would be difficult to handle by a user. In addition, each developer, when replaced, needs to be matched with properties of the photoconductor. Further, the photoconductor, when replaced, needs to be aligned with each developer.